Silver has long been known to be useful as a conductive material and for its antimicrobial effect. The antimicrobial properties of silver have been known for several thousand years. The general pharmacological properties of silver are summarized in “Heavy Metals”—by Stewart C. Harvey and “Antiseptics and Disinfectants: Fungicides; Ectoparasiticides”—by Stewart Harvey in The Pharmacological Basis of Therapeutics, Fifth Edition, by Louis S. Goodman and Alfred Gilman (editors), published by MacMillan Publishing Company, NY, 1975. It is now understood that the affinity of silver ion to biologically important moieties such as sulfhydryl, amino, imidazole, carboxyl and phosphate groups are primarily responsible for its antimicrobial activity.
The attachment of silver ions to one of these reactive groups on a protein results in the precipitation and denaturation of the protein. The extent of the reaction is related to the concentration of silver ions. The interaction is primarily with the proteins in the interstitial space when the silver ion concentration is low; the interaction is with the membrane proteins and intracellular species when the silver ion concentration is high. The diffusion of silver ion into mammalian tissues is self regulated by its intrinsic preference for binding to proteins as well as precipitation by the chloride ions in the environment. Thus, the very affinity of silver ion to a large number of biologically important chemical moieties (an affinity which is responsible for its action as an antimicrobial agent) is also responsible for limiting its systemic action—silver is not easily absorbed by the body. This is a primary reason for the tremendous interest in the use of silver containing species as an anti-microbial i.e. an agent capable of destroying or inhibiting the growth of microorganisms, including bacteria, yeast, fungi and algae, as well as viruses.
In addition to the affinity of silver ions to biologically relevant species, which leads to the denaturation and precipitation of proteins, it is known that some silver compounds having low ionization or dissolution ability function effectively as antiseptics. Distilled water in contact with metallic silver becomes antibacterial, even though the dissolved concentration of silver ions is less than 100 ppb. There are numerous mechanistic pathways by which this oligodynamic effect is manifested, that is, by which silver ion interferes with the basic metabolic activities of bacteria at the cellular level, thus leading to a bacteriocidal and/or bacteriostatic effect.
A detailed review of the oligodynamic effect of silver can be found in “Oligodynamic Metals” by I. B. Romans in Disinfection, Sterlization and Preservation, C. A. Lawrence and S. S. Bloek (editors), published by Lea and Fibiger (1968) and “The Oligodynamic Effect of Silver” by A. Goetz, R. L. Tracy and F. S. Harris, Jr. in Silver in Industry, Lawrence Addicks (editor), published by Reinhold Publishing Corporation, 1940. These reviews describe results that demonstrate that silver is effective as an antimicrobial agent towards a wide range of bacteria. However, it is also known that the efficacy of silver as an antimicrobial agent depends critically on the chemical and physical identity of the silver source. The silver source may be silver in the form of metal particles of varying sizes, silver as a sparingly soluble material such as silver chloride, silver as a highly soluble salt such as silver nitrate, etc. The efficiency of the silver also depends on i) the molecular identity of the active species—whether it is Ag+ ion or a complex species such as (AgCl2)−, etc., and ii) the mechanism by which the active silver species interacts with the organism, which depends on the type of organism. Mechanisms may include, for example, adsorption to the cell wall which causes tearing; plasmolysis where the silver species penetrates the plasma membrane and binds to it; adsorption followed by the coagulation of the protoplasma; or precipitation of the protoplasmic albumin of the bacterial cell, etc. The antibacterial efficacy of silver is determined by the nature and concentration of the active species, the type of bacteria, the surface area of the bacteria that is available to interaction with the active species, the bacterial concentration, the concentration and/or the surface area of species that could consume the active species and lower its activity, the mechanisms of deactivation and so on.
It is clear from the literature on the use of silver based materials as antibacterial agents that there is no general procedure for precipitating silver based materials and/or creating formulations of silver based materials that would be suitable for all applications. Since the efficacy of the formulations depends on so many factors, there is a need for i) a systematic process for generating the source of the desired silver species, ii) a systematic process for creating formulations of silver based materials with a defined concentration of the active species; and iii) a systematic process for delivering these formulations for achieving predetermined efficacy. It is particularly a need for processes which are simple and cost effective.
There is also a need for good conductive materials. Substrates such as polymeric films or glass having an indium tin oxide (ITO) coating thereon are widely used in display devices. The requirements of such a coating are good transparency and electric conductivity. ITO coated substrates are used in applications which include touch panel devices. Touch panel devices have two opposing surfaces of the ITO films separated by spacers. Contact between the two surfaces is made when the front surface is depressed. The location of the input is decoded by an electronic interface. LCD devices include an array of transparent ITO electrodes. The electrodes are fabricated by patterning ITO coating on the substrate. In Electro-Luminescence (EL) displays electricity is converted to light. EL displays have a light-emitting layer sandwiched between two electrodes, one of which is ITO. There are a number of other applications using ITO coatings.
With the proliferation of portable electronic devices such as pagers, phones and notebook computers, ruggedness becomes an important factor in choosing a conductive coating. Since an ITO coating is relatively brittle it is highly desirable to find a more rugged conductive coating to replace ITO.
Silver is known to be an excellent conductor. If silver particles are small enough that they do not block significant amount of light and if particles are interconnected in a coating, it is possible to have a coating of silver particles on a substrate that exhibits high electric conductivity, good transparency and ruggedness.
F. Fievet, et al. disclosed a method in 1989 that used a so-called polyol process to make metallic micro particles (“Solid State Ionics”, Volume 32/33, (1989)). The polyol method mixes metal precursor in ethylene glycol or tetra-ethylene glycol and then heats the mixture to 120 to 200 C for one to three hours. The polyol process relies on ethylene glycol and high temperature for the formation of the small metal particles. P. Toneguzzo, et al. disclosed in 1998 a refined polyol method for synthesizing nano-size polymetallic particles. (“Advanced Materials”, Volume 10, (1998). They used silver or platinum ion as a nucleation agent and then grew other metals to the nuclei. P. Raveendran, et al. disclosed in 2003 a method to make silver nanoparticles in water (“Journal of American Chemical Society”, Volume 125, (2003)). The aqueous method used β-D-glucose to reduce silver nitrate solution in the presence of starch. U.S. Pat. No. 6,676,727 describes a method of preparing small particles of copper and aluminum by evaporating the metals and collecting the vaporized metal in liquid.
All these methods make metal particles that suspend freely and independently from one another. Intrinsic association of metal particles is important for conductive coating applications. It is clear from the literature on the preparation of metal particles that there is no method for preparing associated small particles.
U.S. Pat. No. 4,677,143 describes an antimicrobial composition comprising a binder and an antimicrobial metal compound, said binder having a sufficiently low dielectric constant that when coated allows said antimicrobial compound to form a chain-like structure for the passage of silver ions.
There is still needed a simple and cost effective method for manufacturing silver particles that are highly effective as antimicrobials and conductive materials.